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<TITLE>80386 Programmer's Reference Manual -- Opcode DEC</TITLE>
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<H1>DEC -- Decrement by 1</H1>

<PRE>
Opcode    Instruction        Clocks   Description

FE /1     DEC r/m8           2/6      Decrement r/m byte by 1
FF /1     DEC r/m16          2/6      Decrement r/m word by 1
FF /1     DEC r/m32          2/6      Decrement r/m dword by 1
48+rw     DEC r16            2        Decrement word register by 1
48+rw     DEC r32            2        Decrement dword register by 1
</PRE>

<H2>Operation</H2>

<PRE>
DEST := DEST - 1;
</PRE>

<H2>Description</H2>

DEC subtracts 1 from the operand. DEC does not change the carry flag.
To affect the carry flag, use the 
<A HREF="SUB.htm">SUB</A> instruction with an immediate
operand of 1.

<H2>Flags Affected</H2>

OF, SF, ZF, AF, and PF as described in <A HREF="appc.htm">Appendix C</A>.

<H2>Protected Mode Exceptions</H2>

#GP(0) if the result is a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

<H2>Real Address Mode Exceptions</H2>

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

<H2>Virtual 8086 Mode Exceptions</H2>

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


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<B>up:</B> <A HREF="c17.htm">
Chapter 17 -- 80386 Instruction Set</A><BR>
<B>prev:</B><A HREF="DAS.htm"> DAS Decimal Adjust AL after Subtraction</A><BR>
<B>next:</B><A HREF="DIV.htm"> DIV Unsigned Divide</A> 
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